1. Field of the Invention
The present invention relates to a novel electrode for a lithium secondary battery which has high input-output performance and is advantageous for use typically in hybrid electric vehicles.
2. Description of Related Art
There have been recently made demands to reduce emission of carbon dioxide for protection of the environment and for prevention from global warming. Hybrid electric vehicles (HEVs) and electric vehicles (EVs) have been commercialized as devices for reducing carbon dioxide emission.
Secondary batteries which can undergo a number of charging/discharging cycles are essential devices as power sources for motor-driven vehicles.
Among them, lithium secondary batteries (lithium ion secondary batteries) receive attention as power sources (energy sources) for such motor-driven vehicles because of their high operating voltages and capability of generating a high output.
Lithium secondary batteries for use as power sources for HEVs and EVs require characteristic properties different from those of lithium secondary batteries for use typically in household appliances such as mobile phones and notebook computers.
Specifically, the lithium secondary batteries for HEVs require characteristic properties such as charging-discharging behavior at a high rate, operability at low temperatures of 0° C. or lower, and storage life and cycle life in an environment at high temperatures of higher than 50° C. Among them, battery life at high temperatures is important, because automobiles have product lifecycles longer than those of household appliances and are used in a high-temperature environment.
A carbon material such as graphite is mainly used as a negative-electrode active material for lithium secondary batteries. When this negative-electrode active material is used in a high-temperature environment, delamination between the negative-electrode active material and a copper current collector often occurs and worsens, because there is a difference in coefficient of linear expansion between the negative-electrode active material and the copper current collector and this causes a stress.
In addition, when the lithium secondary batteries are charged and discharged, positive and negative electrodes thereof intercalate and release lithium ions and thereby expand and contract.
The expansion and shrinkage of the electrodes can reduce adhesion between the positive-electrode active material and the current collector or between the negative-electrode active material and the current collector and can thereby cause delamination of the positive-electrode active material or the negative-electrode active material from the current collector.
Independently, the use of silicon (Si), tin (Sn) or another metal capable of forming an alloy with lithium and having a large theoretical capacity as a negative-electrode active material has been studied, for achieving larger capacities of lithium secondary batteries.
When the metal capable of forming an alloy with lithium is used in the negative-electrode active material, the resulting negative-electrode active material shows a volume change larger than that of the carbon material, and the delamination of the negative-electrode active material becomes a more significant problem.
Several possible solutions to these problems have been proposed as below.
Japanese Patent Laid-Open No. 2005-332797 (Patent Literature 1) proposes a technique of using an electrode for a lithium secondary battery, in which active material particles are arranged on a current collector, the active material particle being directly bonded to a surface of the current collector in a state where the bottom of the active material particle is imbedded in a concave portion formed on the surface of the current collector.
Japanese Patent Laid-Open No. 2009-146752 (Patent Literature 2) proposes a technique of using a current collector for a lithium ion secondary battery, in which adhesive resin particles are formed on a surface thereof, and a part of the resin particles is exposed from the surface.